The present invention relates to a method and apparatus for generating optical information, and more particularly to a method and apparatus for generating optical information, which is suitable for an optical recorder such as a laser printer.
Since a conventional laser printer uses a rotating polygon mirror it has been difficult to make it in a compact and solid form. A compact apparatus which used a spatial modulation device has been proposed. (See Proceedings of SPIE (The Society of Photo-Optical Instrumentation Engineers), Vol. 388, p 46). A principle of operation of the apparatus is explained with reference to FIGS. 14, 15 and 16.
FIG. 14 shows an electrode wire pattern formed on a LiNbO.sub.3 crystal 1. The electrode wire pattern comprises a plurality of electrode wires 2. FIG. 15 is a side elevational view of the electrode wires 2 of FIG. 14. By applying a positive or negative voltage to the electrode wires 2, a fringing electric field 8 is generated near the surface in the crystal. If the adjacent electrode wires 2 are of the same potential, no electric field is generated between those electrode wires. Crystal areas between the electrode wires are information areas. In FIG. 15, information "101" is generated between the electrode wires 2 depending on whether the fringing field is generated or not. FIGS. 16A and 16B show a plan view and side elevational view of a conventional spatial modulation device, respectively. When an input laser beam 3 reflects on the surface of the crystal 1, it is phase-modulated by an electro-optical effect due to the fringing electric field shown in FIG. 15. The information generated in the crystal 1 is focused onto a screen 6 by focusing lens 4. Since the information generated in the crystal 1 is phase-modulated, it is necessary to convert the light phase pattern to a light intensity pattern. To this end, a Schlieren optics is used in FIG. 16. A laser beam which passed through the information area having no fringing electric field is blocked by a stopper 5, but since a laser beam which passed through the information area having the fringing electric field is phase-modulated, a light spot spreads on a focal plane of the focusing lens 4 and a light component spread beyond the stopper 5 reaches the screen 6.
In the spatial modulation device of FIG. 16, positional adjustment of the stopper 5 and shape of the stopper 5 largely affect to a signal-to-noise ratio of the information on the screen 6. If the stopper 5 is laterally or longitudinally moved even slightly, the light may be leaked. Thus, the shape and size of the stopper 5 largely affect to the formation of the information.
Further, this system needs high cost because a large crystal is required, and an optical system extending from the laser light source to the crystal is also of large size in order to form the laser beam 3 to be applied to the crystal into a beam shape.